SSC CGL Study Notes: General Awareness (PHYSICS - KINEMATICS)

General Awareness Notes for SSC GGL Exam 2017-18

PHYSICS : KINEMATICS


Kinematics: Kinematics is branch of mechanics which deals with the study of motion of the objects without taking into account the cause of their motion.

Rest and Motion

  • An object is said to be at rest if it does not change its position which respect to its surroundings with time and said to be in motion if it changes its position with respect to its surrounding with time.
  • Rectilinear motion moving car on horizontal road, motion under gravity etc.
  • Angular motion such as particle going on a circle, projectile motion, rotation of machine shaft etc.
  • Rotational motion such as motion of a fan.
  • It an object travels equal distances in equal intervals of time, then it is said to be in uniform motion.
  • It an object travels unequal distances in equal intervals of time, then it is said to be in non-uniform motion.

Circular Motion

  • The motion of an object along a circular path is called circular motion.
  • Circular motion with a constant speed is called uniform circular motion.
  • The direction of motion at any point in circular motion is given by the tangent to the circle at that point.
  • In uniform circular motion, the velocity and acceleration both changes.
  • In case of non-uniform circular motion, the speed changes from point to point on the circular track.

Centripetal Acceleration
During circular motion an acceleration acts on the body towards the centre, called centripetal acceleration.
The direction of centripetal acceleration is always towards the centre of the circular path.

Force: It is an external push or pull with can change or tries to change the state of rest or of uniform motion. SI unit is newton (N) and CGS unit is dyne. 1 N = dyne. If sum of all the forces acting on a body is zero, then body is said to be in equilibrium.

Centripetal Force: During circular motion a force always acts on the body towards the centre of the circular path, called centripetal force.

Centrifugal Force: In circular motion we experience that a force is acting on us in opposite to the direction of centripetal force called centrifugal force. This is an apparent force or imaginary force and also called a pseudo force. Applications of centripetal and centrifugal forces

  • Cyclist inclined itself from vertical to obtain required centripetal force. To take a safe turn cyclist slower down his speed and moves on a path of larger radius.
  • Roads are banked at turns to provide required centripetal force for taking a turn.
  • For taking turn on a curved road, the frictional force is acting between the tyres of the vehicle and the road acts as centripetal force.
  • If a bucket containing water is revolved fast in a vertical plane, the water may not fall even when bucket is completely inverted because a centrifugal force equal or greater than the weight of water pushes the water to the bottom of the bucket.
  • For orbital motion of electrons around the nucleus electrostatic force of attraction is acting between the electrons and the nucleus as centripetal force.
  • Cream is separated from milk when it is rotated in a vessel about the same axis. During rotation lighter particles of cream experience a lesser force than the heavier particles of milk.
  • For revolution of the earth around the sun, gravitational force of attraction between the earth and the sun acts as centripetal force.

Newton's Laws

Newton's First Law: A body continues in its state of rest or of uniform motion in a straight line unless an external force acts on it. It is based on law of inertia. Inertia is the property of a body by virtue of which is opposes any change in its state of rest or of uniform motion in a straight line.

Inertia of Rest

  • When a bus or train at rest starts, to move suddenly, the passangers sitting in it jerk in backward direction due to their inertia of rest.
  • The dust particles come out from a carpet when it is beaten with a stick due to their inertia of rest.
  • A passenger jumping out from a rapidly moving bus or train is advised to jump in forward direction and run forward for a short mile due to inertia of rest.

Inertia of Motion: When a running bus or train stops suddenly, the passengers sitting in it jerk in forward direction due to inertia of motion
Momentum:
The momentum of a moving body is equal to the product of its mass and its velocity.
Newton's Third Law:
For every action, there is an equal and opposite reaction and both act on two different objects. Rocket is propelled by the principle of Newton's third law of motion.

Impulse

  • A large force which acts on a body for a very short interval of time and produces a large change in its momentum is called an impulsive force.
  • Its unit is newton-second.
  • A fielder lowers its hand when catching a cricket ball because by lowering his hands, he increases the time of contact for stopping the ball and therefore fielder has to apply lesser force to stop the ball. The ball will also exert lesser force on the hands of the fielder and the fielder will not get hurt.
  • Wagons of a train are provided with the buffers to increase the time of impact during jerks and therefore, decreases the damage. The vehicles like scooter, car, bus, truck etc. are provided with shockers.

Friction: Friction is a force which opposes the relative motion of the two bodies when one body actually moves or tries to move over the surface of another body. The cause of friction is the strong atomic or molecular forces of attraction acting on the two surfaces at the point of actual contact.

Application of Friction

  • A ball bearing is a type of rolling-element that uses balls to maintain the separation between the bearing races. The purpose of a ball bearing is to reduce rotational friction and to support loads (weight).
  • Friction is necessary for walking, to apply brakes in vehicles, for holding nuts and bolts in a machinery etc.
  • Friction can be decreases by polishing the surfaces by using lubricants or by using ball bearings.
  • Tyres are made of synthetic rubber because its coefficient or friction with road is larger and therefore, large force of friction acts on it, which stops sliding at turns.
  • The tyres are threading which also increases the friction between the tyres and the road.
  • When pedal is applied to a bicycle, the force of friction on rear wheel is in forward direction and on front wheel is in the backward direction.

Lever: It is a simple machine in which a straight or inclined rod is made to turn or rotate at a point freely or independently. There are three points related to lever namely load, effort and fulcrum.

  • Load The weight carried by the lever is called load.
  • Effort To operate lever, the force applied externally is called effort.
  • Fulcrum The fixed point about which the rod of lever moves independently is called fulcrum.

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